1. Field of the Invention
The present invention relates generally to multi-carrier spread spectrum communications.
2. Related Art
Spread spectrum (SS) systems have proven useful in a variety of applications, including wired and wireless communications, ranging, radar, and synchronization. All of these applications may suffer from interference that is ameliorated by the use of SS techniques. SS operates by greatly expanding the frequency spectrum of the information-containing signal. This expansion is controlled by a spreading code. For example, in direct sequence spread spectrum (DS-SS) the information signal is multiplied by a high rate spreading code. The high rate spreading code creates the wide bandwidth transmit signal. Multi-carrier spread spectrum (MC-SS) is an alternative to the conventional DS-SS and frequency hopping spread spectrum (FH-SS) techniques. MC-SS provides a number of distinct advantages over conventional SS. For example, U.S. Pat. No. 5,521,937 issued to Kondo et al. discloses a MC-SS system having resistance to multipath fading and narrow-band interference. MC-SS systems can also provide improved resistance to partial-band interference and jamming.
The improved performance of MC-SS over conventional SS is obtained by transmitting each symbol simultaneously across several sub-carrier bands, where the signal on each sub-carrier band is a conventional (although possibly lower bandwidth) spread spectrum signal. At the receiver, the signals from each sub-carrier band are processed and combined. It is difficult to combine the sub-carriers and maintain good performance, since the optimum weighting of the sub-carriers depends on the per sub-channel channel gain, interference/jamming statistics, and noise statistics. For example, U.S. Pat. No. 5,521,937 discloses a maximum ratio combiner (MRC). The MRC combines the sub-carriers by estimating the signal to noise ratio (SNR) on each sub-carrier band, linearly scaling the signal from each sub-carrier band proportionally to the SNR of that sub-carrier band, and then summing all the channels. The MRC combiner must, however, estimate the SNR for each sub-carrier band. Any errors in the estimate of the SNR result in degradation of performance relative to an optimal receiver. Accurate estimation of SNR has proven difficult to achieve in practical systems.